Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <39d17db6-0f8a-0e54-289b-85b9baf1e936@redhat.com>
Date:   Wed, 16 Feb 2022 20:24:12 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.4.0
Content-Language: en-US
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Oded Gabbay <oded.gabbay@gmail.com>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, Jan Kara <jack@suse.cz>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Maya B . Gokhale" <gokhale2@llnl.gov>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Marty Mcfadden <mcfadden8@llnl.gov>,
        Kirill Shutemov <kirill@shutemov.name>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Xu <peterx@redhat.com>
References: <20200917112538.GD8409@ziepe.ca>
 <CAHk-=wjtfjB3TqTFRzVmOrB9Mii6Yzc-=wKq0fu4ruDE6AsJgg@mail.gmail.com>
 <20200917193824.GL8409@ziepe.ca>
 <CAHk-=wiY_g+SSjncZi8sO=LrxXmMox0NO7K34-Fs653XVXheGg@mail.gmail.com>
 <20200918164032.GA5962@xz-x1> <20200918173240.GY8409@ziepe.ca>
 <20200918204048.GC5962@xz-x1>
 <0af8c77e-ff60-cada-7d22-c7cfcf859b19@nvidia.com>
 <20200919000153.GZ8409@ziepe.ca> <20200921083505.GA5862@quack2.suse.cz>
 <20200921120301.GD8409@ziepe.ca>
 <CAFCwf10eNmwq2wD71xjUhqkvv5+_pJMR1nPug2RqNDcFT4H86Q@mail.gmail.com>
 <CAHk-=wgLk5xWj8v6mQXaWL0zWAf2cis8EpPkQUuEZ7K3LxVQHQ@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
In-Reply-To: <CAHk-=wgLk5xWj8v6mQXaWL0zWAf2cis8EpPkQUuEZ7K3LxVQHQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 16.02.22 20:04, Linus Torvalds wrote:
> [ Added David Hildenbrand to the participants. David, see
> 
>     https://bugzilla.kernel.org/show_bug.cgi?id=215616
> 
>   for details ]
> 

Thanks for sharing.

> On Wed, Feb 16, 2022 at 8:59 AM Oded Gabbay <oded.gabbay@gmail.com> wrote:
>>
>> All the details are in the bug, but the bottom line is that somehow,
>> this patch causes corruption when the numa balancing feature is
>> enabled AND we don't use process affinity AND we use GUP to pin pages
>> so our accelerator can DMA to/from system memory.
> 
> Hmm. I thought all the remaining issues were related to THP - and
> David Hildenbrand had a series to fix those up.


What I shared so far recently [1] was part 1 of my COW fixes to fix the
COW security issues -- missed COW. This fixes 1) of [2].

Part 2 is around fixing "wrong COW" for FOLL_PIN a.k.a. memory
corruption. That's essentially what PageAnonExclusive() will be all
about, making sure that we don't lose synchronicity between GUP and mm
due to a wrong COW. This will fix 3) of [2]

Part 3 is converting O_DIRECT to use FOLL_PIN instead of FOLL_GET to
similarly fix "wrong COW" for O_DIRECT. John is working on that. This
will fix 2) of [2].


> 
> The fact that it also shows up with numa balancing is a bit
> unfortunate, because I think that means that that patch series may not
> have caught that case.
> 
> That said - what does "we use GUP to pin pages" mean? Does it actually
> use the pinning logic, or just regular old GUP?

If it uses FOLL_PIN it might be handled by part 2, if it uses O_DIRECT
magic it might be covered by part 3. If neither of both, more work might
be needed to convert it to FOLL_PIN, as with the new COW logic we won't
be able to have the same guarantees for FOLL_GET as we'll have for
FOLL_PIN (which is a difference to our original plans to fix it all [3]).


[1] https://lkml.kernel.org/r/20220126095557.32392-1-david@redhat.com
[2]
https://lore.kernel.org/all/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@redhat.com/
[3] https://lore.kernel.org/all/20211217113049.23850-1-david@redhat.com/T/#u

-- 
Thanks,

David / dhildenb